Liquid crystal display (LCD) devices contain a liquid crystal material located between a pair of electrodes facing each other. Information written to the display is imaged by creating optical changes in the liquid crystal material by applying a voltage to selected portions of the electrodes.
Conventional LCDs typically use transparent conductive films, such as indium/tin oxide (ITO) for the electrodes. The sheet resistivity of these transparent films used for twisted nematic (TN) and super twisted nematic (STN) displays is typically about 100 Ohms per square. For active matrix displays with video graphic capability, sheet resistivity of less then 10 Ohms per square is required and a resistivity of less than 1 Ohm per square is preferred. When glass or quartz is used as a display substrate material, current techniques require that the substrate be elevated above 300.degree. C. to optimize the film's crystal structure and, thus, the oxide state, in order to attain the specific resistance of 1.times.10(-4) Ohm-cm. Still, even when using this technique, film thicknesses on the order of 10,000 .ANG.ngstroms (1 micron) are needed in order to achieve a sheet resistivity of 1 Ohm per square. At these thicknesses, the tradeoff between conductivity and transparency becomes a concern, because the transparency is now less than 75%, which affects the contrast of the LCDs. Also, the increased thickness needed in order to achieve the high conductivity electrode film begins to have an effect on the gap in the LCD. With gap sizes becoming smaller and smaller and now currently approaching four microns, a metal thickness of greater than one micron on each of the two substrates begins to have a significant effect on the resultant gap of the finished display. Clearly, further reductions in gap size will not be attainable with the current state of technology. Larger displays having even more pixels will require even lower conductivity, thus, thicker electrode material. In addition, the decrease in transparency and reduced contrast due to this thick electrode material now begins to take its toll. Further, the need to use a high-temperature processing step to attain films with acceptable conductivity limits the type of substrate materials which can be used to manufacture displays. For example, it requires that a material such as glass or quartz be used, as opposed to cheaper and lower melting materials such as plastic.
As the size of the LCD increases, the glass is subject to sagging in the center, and also becomes expensive. In order to eliminate these shortcomings, the glass substrates have been replaced with a polymer or plastic substrate which is flexible, light, strong, and more easily formed into shapes. However, when sealing these types of displays with conventional adhesives, epoxies for example, insufficient sealing or adhesion to the polymer is noticed. The use of spherical spacers to maintain the gap is problematical, and as the size of the display increases or as the gap between the electrodes decreases, the requirements placed on the spherical spacers become more and more stringent. Also, in the case of large LCDs, for example greater than 50 mm.times.50 mm, deflection at the center of the plastic display is far worse than in glass displays because of the non-rigid nature of the plastic substrates. Even with the spherical spacers, it is difficult to continually maintain an accurate gap.
Clearly, it would be a desirable improvement if an LCD could be created that utilizes lower cost substrates, such as plastic, on very large displays, and could maintain an extremely uniform and extremely small gap between the two substrates. In addition, it would also be desirable if the electrode pattern could have increased conductivity.